1. Field of the Invention
The present invention relates to a thermal gas flow sensor in which a heating resistor is formed of a semiconductor thin film.
2. Description of the Related Art
Since thermal gas flow sensors can directly detect a gas mass flow, the thermal gas flow sensors are widely used as a flow meter. Such a flow meter is provided in an electronic control fuel injection device in an internal-combustion engine for a vehicle or the like in order to detect an intake air mass. Further, this type of flow meter is used as a flow meter for various gases used in semiconductor fabrication or hydrogen and oxygen used in a fuel cell.
Among these, a gas flow sensor manufactured by a semiconductor micromachining technology is particularly attractive because of its cost reduction and low operating power.
A conventional gas flow sensor using a semiconductor substrate is described in Japanese Patent No. 2880651 and Japanese Patent No. 3698679. In such documents, a heating resistor is formed of polycrystalline silicon (polysilicon) as a substitute for the conventionally used platinum because of advantages of the polysilicon in heat resistance and material cost.